This invention relates generally to a system for transmitting data in MPEG (Motion Picture Experts Group) packet form and specifically to a system for increasing the efficiency of such transmissions. The MPEG packets may comprise encoded video, audio or auxiliary data.
According to the MPEG standard, variable length packetized elementary streams (PES) of data are arranged into fixed length transport packets for transmission. Each MPEG packet comprises a 4 byte header followed by 184 bytes of payload (188 byte packet). The header includes a sync byte (corresponding to 47 HEX) and a 13 bit packet identifier (PID). The 188 byte transport packets are multiplexed for transmission over any suitable transmission medium. The proposed United States standard for advanced television (ATV), which includes both high definition television (HDTV) and lower than HDTV resolution signals, contemplates the transmission of MPEG transport packets using a digital multilevel VSB transmission system. The VSB transmission system in both terrestrial and 2/4/8/16-level cable modes is characterized by a data frame illustrated in FIG. 1. The frame comprises two data fields of 313 data segments each, with each data segment comprising 832 multilevel symbols. The first data segment in each field comprises a data field sync segment and each data segment is headed by a four symbol data segment sync followed by 828 data and FEC (forward error correction) symbols. The field and segment sync symbols facilitate recovery of the data in the ATV receiver and provide timing signals that identify the beginning of each frame and segment. Each field sync segment also includes information identifying the VSB mode.
The transmission is via suppressed carrier modulation. Three hundred and ten kilohertz from the lower band edge, a small DC pilot is added to the signal for use by the VSB receiver in achieving carrier lock. All payload data is randomized to insure that random data is transmitted even when constant data is being supplied to the system. The data and FEC bytes are interleaved for added protection against burst errors.
The symbol rate is 4.5/286 MHz.times.684 which is approximately 10.76 MHz. It will be appreciated that the first term, 4.5/286 is the NTSC horizontal scan rate. All transmitted signals are multilevel. The sync symbols are always 2-level (binary). In the terrestrial mode, 8 level trellis-coded symbols (3 bits per symbol) are transmitted while for the cable mode, 16/8/4/2-level symbols (4/3/2/1 bits per symbol, respectively) are used. These are referred to as trellis coded 8 VSB for the terrestrial mode and 16/8/4/2 VSB for the cable mode. In the 16 VSB cable mode, each segment includes four 2-level sync symbols plus 414 data bytes (828.times.4.div.8). In the 8 VSB cable and trellis modes, each segment includes four 2-level sync symbols plus 310.5 data bytes. In the 4 VSB cable mode, each segment includes four 2-level sync symbols plus 207 data bytes. In the 2 VSB cable mode, each segment includes four 2-level sync symbols plus 103.5 data bytes.
The use of MPEG transport packets for the VSB transmission results in an unnecessary redundancy and a reduction of data capacity since both MPEG sync bytes and VSB data segment sync symbols are included. The purpose of the MPEG sync byte is to identify the beginning of a packet and the same information can be derived from the segment sync in VSB. Moreover, the information can be derived very conveniently due to the VSB parameters, i.e. symbols/segment and bits/symbol.